There are a variety of situations in which it is desired to expand a metal tube radially to form a tight, leak-free joint. For example, large heat exchangers, particularly the type used as steam generators in nuclear power plants, often employ a tube sheet, which is a steel plate several feet thick, through which hundreds of stainless steel or carbon steel tubes must pass. The tube sheet is initially fabricated with bores of a suitable diameter in which the tubes are inserted. The tubes are then expanded against the sides of the bores by plastic deformation to seal the small crevices that would otherwise exist around the tubes. If these crevices were allowed to remain, they could collect corrosive agents, and would, therefore, decrease the reliable and predictable life-expectancy of the equipment.
In general, the most effective state of the art apparatus for difficult swaging jobs that require high magnitude forces employ a mandrel that is inserted in the tube. Pressurized hydraulic fluid is then introduced to an annular volume or pressure zone between the mandrel and the tube, forcing the tube to expand radially.
Each such mandrel requires two seals that define the axial boundaries of the pressure zone. The construction of these seals presents unusually difficult technical problems because materials that have the necessary elastic properties to prevent leakage of the hydraulic fluid tend to extrude into any available gaps or small volumes and deform inelastically, thus damaging the seal.
It has been found to be desirable to use two element seals. The primary seal element, which comes into direct contact with the hydraulic fluid is relatively soft. Usually, a rubber O-ring is used. An adjacent element, referred to as a backup member, is more rigid but still behaves elastically at the high pressures applied to it. A polyurethane ring is well suited to this use. It is compressed axially by the swaging pressure and expands radially as the tube expands.
While a backup member prevents extrusion damage to the primary seal element, it has been found that at high swaging pressures the backup member itself may be inelastically deformed by extrusion into an adjacent annular gap on the low pressure side of the seal that necessarily widens as the tube expands.
An objective of the present invention is to provide an improved swaging apparatus in which the problem of destructive inelastic extrusion of the elastic element or elements of the seal is minimized or eliminated.